This invention relates generally to small-sized electronic timepieces having liquid crystal displays adapted to be selectively illuminated by a light source and especially to an electronic wristwatch having a single power source for energizing the timekeeping circuitry and illuminating the light source.
Solid-state electronic timepieces having digital displays are of two types, the light-emitting type, wherein light-emitting diodes (LED) or the like are utilized as display elements or the passive type wherein liquid crystal elements are utilized as the display elements. Those which utilize light-emitting elements have the disadvantage of requiring extremely large amounts of current to drive same and therefore require either a large battery or several large batteries to be incorporated in the timepiece if the display is to be rendered light-emitting for long periods of time. For instance, it would require at least 100 compact button-type batteries to maintain an electronic light-emitting digital display continuously lit for at least a year, the space required to store the batteries rendering the use of such batteries impossible in a small-sized electronic timepiece. To overcome this disadvantage, light-emitting displays are switched on and off when the viewer wants to view the time, a display switch being provided on the wristwatch to allow the light-emitting elements to become energized. Such a use of light-emitting elements is known as the "demand" method. It is appreciated that the demand method is inconvenient to a wearer who is holding packages or other objects in one hand or both hands, as he then cannot determine the time without freeing his hands so as to allow him to actuate the display switch.
Nevertheless, one advantage of light-emitting displays is that the display can be viewed in the nighttime or in very dark places. A passive display, such as one formed of liquid crystals, although providing continuous displays and only utilizing one-ten thousandth the current utilized by a light-emitting display, cannot be read in dark places because it requires a light source to illuminate same. Furthermore, this disadvantage cannot be overcome by providing luminous painted dots, as is done to the dial and hands in a mechanical timepiece, because the use of enough luminous paint to effect a display would exceed acceptable limits of radioactivity and therefore render impossible the use of luminous paint in a passive-type watch.
It has been found effective to utilize a miniature bulb proximate to the liquid crystal display in an electronic wristwatch. However, a second source of energy is required to energize the bulb, and although it would be convenient to utilize a battery in the same manner that a battery is used in addition to a barrel drum as an energy source in conventional mechanical timepieces, such use is not practical in a small-sized electronic wristwatch because of its limited space requirements.